Circuito Superior is the upper trail on the Argentine side of Iguazú Falls, a boardwalk route that takes you across the river islands and along the very lip of several cascades. Instead of looking up at the falls, you look across and down into the water as it breaks away.
It complements the lower circuit rather than replacing it. Do both if you have time: the lower trail gives scale and spray, while the upper circuit gives pattern, movement, and the unnerving sense of standing just before the drop.